The present invention relates to a flux for Pb-free Sn-based alloy solders. More specifically, it relates to a rosin-based flux used a soldering paste for Sn-based alloy solder containing no Pb, so-called Pb-free solders such as Snxe2x80x94Ag, Snxe2x80x94Agxe2x80x94Bi or Snxe2x80x94Agxe2x80x94Cu based alloys, which will be used in the electronics industries fields in replace to a conventional Snxe2x80x94Pb solder. Further, it relates to a method for preparing said flux for the Pb-free solder. It relates to a soldering flux suitable for so-called Pb-free solder, particularly, for Snxe2x80x94Ag, Snxe2x80x94Agxe2x80x94Bi or Snxe2x80x94Agxe2x80x94Cu based solders, which contain Ag as a constitutional component.
A soldering step is an essential step for electronic circuit manufacturing. In this step, it is necessary to remove a surface oxidized film of a solder alloy itself or a naturally oxidized film on a surface of a copper foil, to be soldered, on a substrate of a printed circuit to make wetting and affinity of the solder with surfaces of metals to be jointed preferable. For a pretreatment to clean this jointing face, soldering paste or that named flux is used. In soldering, melting of the solder alloy and brazing jointing are simultaneously carried out, and thus the flux treatment is conducted when heating up.
Conventionally, for soldering, the Snxe2x80x94Pb alloy was used as a solder material. For soldering using this Snxe2x80x94Pb alloy, since old days, flux, also called soldering paste, such as that prepared by suspending a zinc chloride fine powder in paraffin or that prepared by suspending an ammonium chloride fine powder in rosin was used. As a rule, the inorganic component suspended in this flux (soldering paste) corrodes and breaks the oxide film and on the other hand, an organic component covers the surface of the metal exposed to prevent to contact with air during the step as described above so that it is thought to take a role for preventing repeated formation of the oxide film.
In other words, the oxide film is successfully removed through the following steps. For example, in the flux (soldering paste) prepared by suspending the zinc chloride fine powder in paraffin, the zinc chloride fine powder reacts to tin oxide and the like and paraffin prevents to contact with air. For example, presumed is the following mechanism: zinc chloride reacts to tin oxide to convert to a chloride of tin resulting in an oxide of zinc and thus, corrodes and breaks the oxide film on the Pbxe2x80x94Sn solder alloy.
Meanwhile, in the flux (soldering paste) prepared by suspending an ammonium chloride fine powder in rosin, it may be understood that ammonium chloride is the inorganic component reacting to tin oxide while rosin is the organic component taking a role of preventing to contact with air similarly to paraffin as described above. In the case, for example, ammonium chloride reacts to tin oxide to convert once to tin chloride resulting in dissolution from the surface. Thereafter, a resin acid, abietic acid, and the like, which are main components of rosin, reacts to tin chloride dissolved out as a carboxylic acid to convert it into its carboxylate. On the other hand, a chloride ion species dissociated from tin chloride regenerate ammonium chloride. Such as tin abietate generated is dissolved in rosin molten. As a result, it can be understood that tin oxide is converted into tin abietate to remove the oxide film on the Pbxe2x80x94Sn solder alloy. Also on the surface of copper, it can be interpreted that removal of the oxide film is achieved by a similar reaction.
Consequently, in the flux (soldering paste) prepared by suspending an ammonium chloride fine powder in rosin, ammonium chloride becomes the inorganic component having the action of enhancing the flux activity. The resin acid such as abietic acid contained in rosin is insoluble in water and acid strength thereof is not high. Ammonium chloride, if assumed that it works as ammonia and hydrogen chloride, becomes that showing acid strength. Acidity of rosin itself is yielded from a result of that the resin acid such as abietic acid contained acts as a carboxylic acid which is a proton donor. When ammonium chloride and the like is added, a whole of the flux is realized to enhance the acid strength.
As described above, in the conventional flux, rosin used as the base material has solvent function to disperse fine particles of the inorganic component and in addition, the resin acid, abietic acid, and its relatives, which are main components of rosin, convert the metal oxide of the oxide film into its abietate, and has the solvent function to dissolve the abietate. In addition to this, the resin acid, abietic acid, or its relative has a boiling point as high as 280xc2x0 C. to be suitable for a third role to insulate from air at a temperature such as that near a reflowing temperature to cause the above described reaction. Rosin itself is softened at about 100xc2x0 C. and solid at a room temperature, and therefore, preferable for even uniform dispersion of the fine powder of ammonium chloride to keep a state of dispersion.
Conventionally, the Snxe2x80x94Pb alloy used as the solder material in reflowing may spatter a small amount of lead or lead compound as a fine floating splash. Lead or lead compound such as lead oxide, lead dichloride, lead tetrachloride, and the like are poisonous substances so that even a small amount taken and accumulated by a body for a long time influences badly to health. In the soldering process itself in the electronic circuit manufacture, in order to prevent to expose an operator to the lead or lead compounds, the operation is carried out in a controlled and perfectly designed environment. However, there is an extremely small floating fine powder passed through such control and a problem of disposition of the lead or lead compound removed and collected yet arises. In addition, an electronic circuit shipped as a product is someday disposed and it is anxious that the Snxe2x80x94Pb alloy left therein, in destruction by fire, generates the lead or lead compound similarly to soldering process to cause environmental pollution.
In order to avoid radically the problem described above, use of the Snxe2x80x94Pb alloy as the solder material is stopped and in replacement thereto, use of the Sn-based alloy solder free from Pb (so-called Pb-free solder), in particular, such as Snxe2x80x94Ag, Snxe2x80x94Agxe2x80x94Bi or Snxe2x80x94Agxe2x80x94Cu based solder is increasing. The solder material is the alloy in which Ag, Bi, Cu or the like is added to Sn, the main component, to lower a melting point thereof. The flux effectively removing oxides thereof is required. Conventionally, it has been known that the flux containing rosin as the base material to be used for the Snxe2x80x94Pb alloy causes deterioration of material or disorder in durability, when used for the Pb-free solder, such as Snxe2x80x94Ag or Snxe2x80x94Agxe2x80x94Bi based solder.
The present inventors consider that a flux containing rosin as a base material used for conventional Snxe2x80x94Pb alloys raises such a problem to cause deterioration of material or defect in durability as described above, when used for Pb-free solder, such as Snxe2x80x94Ag or Snxe2x80x94Agxe2x80x94Bi based solder, and therefore, there is necessity to develop a new flux compatible with Pb-free solder, such as Snxe2x80x94Ag or Snxe2x80x94Agxe2x80x94Bi based solder. In addition, in comparison with conventional Snxe2x80x94Pb alloys, the Pb-free solder, such as Snxe2x80x94Ag or Snxe2x80x94Agxe2x80x94Bi based solder has a higher flex temperature and hence, a new flux that maintains a high workability even at such a higher reflowing temperature and does not adversely affect a working environment must be developed.
The present invention solves the problem as described above and an object of the present invention is to provide a new flux having sufficient cleaning function, which causes no deterioration of material or no defect in durability when used for Pb-free solder, such as Snxe2x80x94Ag or Snxe2x80x94Agxe2x80x94Bi based solder and tolerates to be used under a high flex temperature. Also, the object includes further providing a new flux that is free from such as a volatile matter adversely affecting working environment.
The inventors, to solve the problem as described above, studied intensively and found as described later that identifying and removing a causal factor of deterioration of material or defect in durability occurring when conventional flux, of which base material is rosin, is used for Pb-free solder, such as Snxe2x80x94Ag or Snxe2x80x94Agxe2x80x94Bi based solder, allow preparing a rosin-based flux, in which occurrence of deterioration of material or defect in durability is prevented even for Pb-free solder, such as Snxe2x80x94Ag or Snxe2x80x94Agxe2x80x94Bi based solder to complete the present invention.
According to the present invention, a flux for Pb-free Sn-based alloy solder of the first invention is a soldering flux, in which a rosin-based flux comprises a rosin-based base material containing abietic acid as a main component, characterized in that
the flux contains fine powder of an inorganic component having an action of enhancing a flux activity that is added to and dispersed in the rosin-based base material, and
the flux is free from any compound of which a constitutional element is a halide ion.
For example, the flux can be a flux for soldering, characterized in that said fine powder of the inorganic component is fine powder of a solid acid. In addition, it is preferable that the flux is a flux for soldering characterized in that said fine powder of solid acid is a solid acid for which an acid strength has been enhanced or an acid point has been induced by hydrogen reduction. In addition, the flux can be a flux for soldering, characterized in that said fine powder of solid acid is fine powder of a complex that comprises a fine power carrier consisting of an inorganic compound and an inorganic compound other than the inorganic compound used for the fine power carrier, which is supported on the carrier, and the fine powder of said complex, as a whole, exhibits an action as the solid acid.
For example, a more preferable flux for soldering is that in which the fine powder of said complex is fine powder of Al2O3xe2x80x94NiO or/and Al2O3xe2x80x94Cr2O3 in which NiO or Cr2O3 is supported on a fine powder Al2O3 as a carrier.
It is more preferable also that the soldering flux according to the first invention is a flux for soldering, in which said rosin-based base material is denatured rosin that is prepared by hydrodesulfurization treatment of crudely purified rosin in the presence of a desulfurization catalyst.
In addition, a flux for the Pb-free Sn based alloy solder according to the second invention of the present invention is the soldering flux, in which a rosin-based flux contains a rosin-based base material containing abietic acid as a main component, characterized in that said rosin-based base material is denatured rosin prepared by hydrodesulfurization treatment of a crudely purified rosin in the presence of a desulfurization catalyst.
Furthermore, in the case where containing the denatured rosin prepared by hydrodesulfurization treatment as the rosin-based base material, it is more preferable that the soldering flux according to the present invention is the soldering flux, wherein said denatured rosin is denatured rosin prepared by hydrodesulfurization treatment and also a treatment for removal by evaporation of an evaporative organic compound impurity contained in the impurity derived from the crudely purified rosin.
The treatment for removal by evaporation of the evaporative organic compound impurity contained in the impurity derived from the crudely purified rosin means treatment to remove the evaporative organic compound impurity evaporating at the melting temperature of the Pb-free solder such as Snxe2x80x94Ag or Snxe2x80x94Agxe2x80x94Bi based solder or lower.
The process for manufacturing a flux for Pb-free Sn based alloy solder according to the present invention is a process for manufacturing a flux, which comprises a rosin-based base material containing abietic acid as a main component and fine powder of an inorganic component having an action of enhancing a flux activity, characterized in that at least
comprising a step for preparing denatured rosin by hydrodesulfurization treatment of a crudely purified rosin in the presence of the desulfurization catalyst,
wherein said desulfurization catalyst used is a desulfurization catalyst working also as fine powder of an inorganic component having an action of enhancing a flux activity,
which is, after said step for hydrodesulfurization treatment, used as the fine powder of the inorganic component having an action of enhancing a flux activity as it is. Therefore, it is a process for manufacturing, characterized in that the step for preparing denatured rosin by hydrodesulfurization treatment of a crudely purified rosin in the presence of the desulfurization catalyst and the step for adding and dispersing fine powder of an inorganic component, which has the action enhancing the flux activity, in denatured rosin are integrally operated as described above by using the fine powder of said inorganic component as the desulfurization catalyst used for said hydrodesulfurization treatment.